Wireless charging systems exploit propagation of energy by a radio frequency electromagnetic field between a transmitting unit and a receiving unit, which may be embedded in a chargeable device or may be coupled to such a device when it is required to charge it. The receiving unit has to cope with varying power levels in the electromagnetic field and with varying distances to the transmitting unit.
Commonly the impedance of the receiving unit is varied to compensate for changing power levels and/or changes in the impedance in the receiving antenna that may be caused by varying distances between the receiving and transmitting units. Another possibility is that the receiving device includes a charging management unit for controlling the charging current and voltage throughout the charging process.